Need help repairing couple 'scopes (different faults)

[fezder]

If you find a b-e junction that's zero, it may be the surrounding stuff causing it

right, didn't thought about that...reminds of that signal generator i tried to fix earlier (haven't tossed it yet)
i could measure again, this time in-circuit?
edit: measured, nothing alaming on ok-state

You could TRY to freeze spray the op amp in the power supply with freeze spray when it acts up.

didn't even notice opamp before there, worth a try

I wonder if we should shelve this until the main filter caps are replaced?

sure, no rush with this thing, they're thankfully not too expensive parts. still, i'll buy good quality caps, not crappy-chinese

 

[KeepItSimpleStupid]

There would be no harm done if you hit the OP amp, bridge rectifier and Pass transistor with freeze spray either.

 

[fezder]

Pass transistor

what is it exactly?

 

[throbscottle]

In a linear regulator, it is the transistor which is in series with the load. It acts like a variable resistance to keep the output voltage constant. it "passes" the load current to the load.

 

[KeepItSimpleStupid]

I have another idea: Look at the output of pin #1 of U603 leading up to the fault. It should be an indication of whether the current is increasing on the 12V supply.

 

[fezder]

thanks throb!

I have another idea: Look at the output of pin #1 of U603 leading up to the fault. It should be an indication of whether the current is increasing on the 12V supply.

worth a try, i'll post the result in a while

 

[fezder]

okay, pretty huge difference between okay and faulty state.
video is at from faulty state (link)
https://onedrive.live.com/?cid=4773...228&ithint=video,AVI&authkey=!AOWeesKPIfRQSzM
picture is at okay state.
2 volts/div is at both and dc-selection

 

[fezder]

couple obesrvations using freezer: fault disappears when bridge rectrifiers are hit. also, beam disappears (still backlight is abnormally bright) when opamps are hit (u603)

 

[KeepItSimpleStupid]

OK, replace the bridge and the big caps (3). Buy an OP amp and socket, but don't replace it. Re-convene.

 

[fezder]

figured that would be next step, what do you think, there are two bridges, should both be replaced?
and umm, meanwhile we wait for these parts, should we ''focus'' on other scopes? other had burned resistor, that was sure, in symmetrical section, like in this kikusui's.

 

[KeepItSimpleStupid]

I looked, there are 4 big caps in the power supply. Rate at LEAST 2x, the voltage seen.

What's the p/n of the bridge? Current and PRV?
MAYBE, they need to be upgraded.

Replace both bridges? Dunno. Get the p/n from the component and the original specs.

If you have another scope to try to fix, start another thread.

 

[fezder]

I looked, there are 4 big caps in the power supply. Rate at LEAST 2x, the voltage seen.

cotcha, but i see only 3 ''big'' 2 pcs of 1000uf/35v, and one 47uf/200v (plus 250v/2.2uf so yeah, that makes four.....). I suppose you mean more voltage for those 35v but is it really nececcasy for that 200v? or by voltage seen, did you mean what i see via meter/scope?

What's the p/n of the bridge? Current and PRV?

as far as i can see from schematics, p/n is either 1G4B42 or IG4B42. That shouldn't be issue, as they both return datasheets, only from different manufacturers. Average output current is 1A and PRV is 400v

 

[KeepItSimpleStupid]

The 47 @ 200 V has been replaced. The schematic says 25 V. I'd get a lower tolerance cap there, e.g. +-10% vs -10%+75%

Bridge: PRV of 100-200 should be fine.

IF it gets all fixed, we can select a couple of surge suppressors, TVS diodes, whatever. Not now - they are too expensive.

 

[KeepItSimpleStupid]

Fez: I've invested in a p-touch labelmaker such as: https://www.brother-usa.com/Ptouch/Ptouch_HandHeld/#.VIS3u5H0Fok

I got one that has electrical symbols on it and can do "wire labels". i.e. Flags etc. It has a number of label types. The labels are very durable. Every wall wart gets marked: what it's for. The voltage, the polarity, the current and the size of the plug, e.g. 5.5/2.1 gets marked at the plug end with a wire label. If it's missing on the device itself, it gets added.

I use it for so many things. So, yea, if i replaced something, I might put a label in the case as to what was replaced and when and the original part number etc.

I do the same for battery packs, The purchase month/year and website and the OEM part number and the part number I used.
I do it for labtop batteries too.

When I can, I use a Statler Lumocolor fine marker. You can erase the "permanent" ink with methanol and methanol doesn't harm plastics.

So, a smoke detector battery gets marked when it was replaced last with a Stadler pen.

The last time, I used the labelmaker was yesterday. I bought 3 "German engineered" cobalt step drills (Unibits) in a case and one of the bits won't stay in if inverted. So, after thinking for awhile, I got a 1/4" plumbing tubing to 1/8 NPT adapter and attached it with a plastic thread forming screw. Now, it doesn't fall out. Cobalt can break easily.

 

[fezder]

The 47 @ 200 V has been replaced. The schematic says 25 V. I'd get a lower tolerance cap there, e.g. +-10% vs -10%+75%

youre right! didn't expect that at all, i mean that it have been replaced.

IF it gets all fixed, we can select a couple of surge suppressors, TVS diodes, whatever. Not now - they are too expensive.

hopefully, feels good scope so far, easy to use, overall shape seems good (except couple calibration knobs missing, but they are in ''normal'' selection

but, i'll buy the stuff, right after i get answer to this: opamp is μpc4558, and hard to find excatly that. what are gualifies as replacement? with 4558, i can find LM4558, MC1458-8P

as for stuff found so far, these should be okay right?
https://www.tme.eu/fi/details/ubt1v102mhd8/tht-125c-elektrolyyttikondensaattorit/nichicon/
https://www.tme.eu/fi/details/upw2e470mhd/tht-matalan-imped-elektrolyyttikond/nichicon/
https://www.tme.eu/fi/details/ce-2.2_250pht/tht-105c-elektrolyyttikondensaattorit/#
https://www.tme.eu/fi/details/db103/smdtht-yksivaih-diodisilta-tasasuun/dc-components/#

of course, there are other (perhaps better) places than tme...

 

[fezder]

hey, not bad idea. i have actaully marker like that, it's just bad quality, i don't use it (glue wont stay, and ''stamps'' pop out from tension afterwards....)

 

[KeepItSimpleStupid]

Fine for the caps and the dual OP amp. MC, LM both fine. Get a socket for the OP amp. Machine pin preferable,, nit essential.

 

[fezder]

i do have sockets (thankfully...) but, why socket? easy to replace probadly, but i'we heard it can cause malfunctions

 

[KeepItSimpleStupid]

Can. Your choice. Not planning to replace OP amp anyway. The machine pin sockets seem to be the most reliable.

It depends a lot on your unsoldering skills. Usually you would sacrifice the ic (cut the pins) on removal.

 

[fezder]

I got one that has electrical symbols on it and can do "wire labels". i.e. Flags etc. It has a number of label types. The labels are very durable. Every wall wart gets marked: what it's for. The voltage, the polarity, the current and the size of the plug, e.g. 5.5/2.1 gets marked at the plug end with a wire label. If it's missing on the device itself, it gets added.

oo boy, didn't read that one. i deffinedly want one of those. what model you have? or is it ''obsolete''?